Aircraft having electrical propulsors have been proposed, in which electrical power generated by a gas turbine engine driven electrical generator is used to power an electrical motor, which in turn drives a propulsor such as a fan or propeller to power the aircraft in flight. Studies such as “Distributed Turboelectric Propulsion for Hybrid Wing Body Aircraft” by Hyun Dae Kim, Gerald V Brown and James L Felder, published by the Royal Aeronautical Society, have shown that such systems may provide efficiency savings in some instances, as large numbers of propulsors can be used, which increases the overall efficiency of the aircraft. However, inefficiencies in the electrical network can in some cases negate these advantages. Such inefficiencies include the weight of the associated generators, power electronics devices and motors, and power losses in electrical transmission, frequency control by the power electronics, and conversion between mechanical and electrical power by the generators and motors.
AC electrical generators, motors and transmissions are known to have high transmission and conversion efficiencies. However, in order to provide variable motor speed to in turn provide variable propeller or fan rotational speed (and so provide variable thrust over a range of aircraft forward flight airspeeds), power electronics units are generally required in order manage the frequency of electrical power delivered to the electrical motors to ensure synchronisation of applied and rotor fields, and also to control the applied voltage which can impact motor efficiency. Where the electrical motors comprise synchronous motors, power electronics are also required for motor starting, as large synchronous motors generally have large inertia, and so are not self-starting—due to the requirement for the applied stator field and the rotor speed to be synchronised (otherwise continuous rotation is not achieved and pole slipping or cogging may occur). However, power electronics units are generally heavy, inefficient and expensive. The need for power electronics in the electrical network rated to the full power required by the propulsor driving motors at full thrust may therefore result in disadvantages that outweigh the advantages provided by electrical propulsion.
The present invention seeks to provide an aircraft electrical network for an aircraft and a method of starting an electrical motor powered by the electrical network which seeks to overcome some or all of the above problems.